Perimeter-weighted investment cast metal golf clubs, both wooden and iron shaped clubs, have found increasing popularity in the marketplace replacing solid core woods and forged irons.
These perimeter-weighted clubs have achieved marketplace success for a variety of reasons including a marketplace perception that off-center hits with perimeter weighted club heads yield far better results than with off-center hits on solid wooden club heads and forged irons. Perimeter-weighted club heads usually include a rather thin forward ball in the hitting area bounded by a much heavier wall, either thicker or longer, around the perimeter of the forward wall. In the case of perimeter-weighted metal woods, the perimeter ball has a thickness on the order of 0.060 to 0.090 inches, somewhat less than the forward wall, but because of its large bell-shaped configuration, has considerable weight severely reducing the resiliency of the club face at the perimeter of the forward wall giving the golfer a stiffer feel for off-center hits near the perimeter wall.
In perimeter-weighted irons, the perimeter weight is increased by thickening the perimeter wall both parallel and normal to the face, but in either case the result is the same; i.e., increased club head weight around the perimeter of the forward wall and decreased forward wall thickness in the hitting area.
The combination of a heavy perimeter wall and a thin central wall in the ball striking area produces a result similar to a tennis racquet effect where the ball deflects the forward wall rearwardly at impact hugs the ball for about 15 milliseconds and slings it forwardly off the club face. This is sometimes also referred to as a banjo effect.
While most players sincerely believe these perimeter-weighted woods and irons to be more forgiving for off-center hits, the results produced by off-center hits are inconsistent both in terms of ball velocity and "feel". This is because the deflection of this thin forward wall varies dramatically between impact points at the center of the club and impact points moving outwardly from that center toward the perimeter.
I discovered this phenomenon as a result of testing club face deflection in a perimeter-weighted stainless steel wood while attempting to build electronics into the club face to compute ball distance traveled. I found this computation difficult because club face deflection was at a maximum of 100 um.times.k (where k is the constant), at the club face center while only 20 um.times.k at the perimeter for exactly the same impact force. This drastic reduction in sling shot effect may give the golfer a somewhat more rigid feel for hits near the perimeter wall but also results in a significant velocity decrease because of the reduction in sling shot effect.
Because of this problem, I found it desirable to design a golf club head that would have uniform force deflection characteristics across the entire club face, i.e., not only at the center of the club face but moving outwardly from the center toward the periphery.
Perimeter-weighted investment cast woods and irons have principally found success in the marketplace because of ease of manufacture, and only to a lesser extent the improved off-center hit characteristic. The cost reduction is due to the fact that these perimeter-weighted clubs can be easily investment cast, sometimes referred to as a lost wax process, in which rather complex iron configurations and hollow thin-walled metal woods can be molded utilizing wax patterns frequently formed with inexpensive epoxy thermo-setting masters.
Perimeter-weighted metal woods, and particularly those formed of stainless steel today usually have forward walls on the order of .100 to 0.125 inches in thickness and perimeter walls on the order of 0.050 to 0.080. Bearing in mind that overall club head weight of a metal wood must be in the range of 13 to 13.5 oz. for useability by professionals and amateurs, early attempts at designing investment cast metal woods produced forward walls less than 0.100 inches in thickness and these frequently failed under impact loads. In early efforts to minimize this problem without adding thickness to the forward wall, various shaped partitions were devised across the forward wall to give it added strength. It should be understood, however, that all these attempts at partitioning the forward wall were for the sole purpose of strengthening an already too thin forward wall.
Another distinguishing characteristic in these earlier partitioning efforts is that the investment cast woods were and in fact are presently formed of a main upper body and hosel and a lower sole plate joined together by welding. It is necessary in the investment casting process that a collapsible core piece be provided to form the hollow thin-walled interior since the hollow interior is significantly larger than the sole plate. Because the sole plate covers an opening in the lower part of the body, the core piece is removed by first collapsing it and then pulling it vertically downwardly. This required collapsing and withdrawal movement of the investment casting core piece severely restricts the configuration of any partitioning extending from the forward wall because the core piece must be able to also withdraw from and clear the partitioning also.
An early attempt at perimeter-weighting and front face strengthening is shown in the Raymont, U.S. Pat. No. 3,847,399, although Raymont shows his partitioning in wooden clubs only for hollowed out natural wood bodies. His iron configuration, however, is perimeter-weighted in a style similar to many perimeter-weighted clubs found today with a thin-walled central face and a heavy, thick surrounding perimeter wall. The central area of the forward face is strengthened by rearwardly extending partitions having the configuration of honeycombing. This honeycombing, however, is for the sole purpose of strengthening the forward face and not minimizing the effect of perimeter weighting. Furthermore, this partitioning extends over only the central portion of the club face and not entirely across it. Raymont also envisioned partitions for rigidifying the forward face in wooden clubs, but he only appreciated its application to natural wooden club heads and not to thin wall investment cast metal woods.
Similar strengthening partitioning in golf club heads are shown in the Marker, U.S. Pat. No. 1,592,463; the Tobia, U.S. Pat. No. 1,658,581; the Drevitson, U.S. Pat. No. 1,678,637; the Schaffer, U.S. Pat. No. 2,460,435; the Curley, U.S. Pat. No. 2,592,013; the Mader, U.S. Pat. No. 4,021,047; the Nygren, U.S. Pat. No. 4,076,254; the Zebelean, U.S. Pat. No. 4,214,754; the Motomiya, U.S. Pat. No. 4,438,931; the Hayashi et al. U.S. Pat. No. 4,449,707; the Yamada U.S. Pat. No. 4,535,990; the Teramoto, et al., U.S. Pat. No. 4,645,207; the Straza, et al., U.S. Pat. No. 4,679,792; the Chen, et al., U.S. Pat. No. 4,681,32l, and the Tilley, U.S. Pat. No. 4,730,830. There have also been attempts to provide club heads with rear covers, such as shown in the Nero, U.S. Pat. No. 1,825,244, and the Carlino, U.S. Pat. No. 3,637,218, but these attempts have not been applied to thin walled investment cast club heads, and thus not directed to any of the problems in that casting art.
There have also been attempts to manufacture wooden clubs with narrow faces such as the one shown in the East, U.S. Pat. No. 3,693,978, but these have not suggested shaping the rear wall in a wooden club complementary and parallel to the forward ball striking face.